Generally, the advance in industrialization is accompanied by the increase in the production of industrial machines including diesel engines and automobiles, increasing the consumption of a diesel oil used as a fuel of the industrial machines and automobiles. Of various fuels produced from crude oil, diesel oil is competitive because of its relatively low cost, but is problematic in that combustion using diesel oil as fuel causes greater pollution than other kinds of fuels.
Much effort has been made to develop a bio-diesel oil as a substitute fuel having similar physical properties to the diesel oil and capable of preventing air pollution in order to overcome such a problem experienced with combustion using the diesel oil.
The bio-diesel oil is an esterified oil produced by reacting a regenerable oil/fat, such as vegetable oil, animal fat, and waste frying oil with alcohol in the presence or absence of a catalyst. Having similar physical properties to light oil used as the fuel for the diesel engines, the bio-diesel oil may be used with light oil, or used alone without using light oil in the diesel engines.
It is necessary to understand some production characteristics of the bio-diesel oil in order to develop a process for effectively producing the bio-diesel oil. Firstly, the production of the bio-diesel oil is characterized in that the bio-diesel oil is produced through a reversible reaction. Accordingly, a batch reactor or a plug flow reactor (PFR) maintaining high concentrations of reactants therein is more preferable than a continuous stirred tank reactor (CSTR) maintaining low concentrations of the reactants therein when it is required to maintain a high reaction rate.
Secondly, polar alcohol is used to produce the bio-diesel oil (various alcohols may be used to produce the bio-diesel oil, but relatively low-priced methanol is most frequently used to practically produce the bio-diesel oil). As described above, alcohol used as one of the reactants is polar liquid, but the oil/fat used as the other reactant is non-polar liquid. Therefore, the two reactants are immiscible with each other, but exist in two separate phases. At this time, the reaction between the two reactants is conducted at only an interface between the two reactants, thereby reducing the reaction rate of the oil/fat and alcohol.
Thirdly, it is preferable that a continuous reactor be used instead of the batch reactor to produce the bio-diesel oil because the bio-diesel oil must be produced as fuel for automobiles in a great amount. Hence, the PFR or CSTR is preferable in comparison with the batch reactor. Taking all the above into consideration, it can be seen that it is necessary to use the PFR to produce the bio-diesel oil in a great amount, and to homogeneously mix the oil/fat with alcohol to increase the reaction rate of alcohol with the oil/fat.
The homogeneous mixing of the oil/fat and alcohol has been developed in the art, as indicated by U.S. Pat. No. 5,514,820, which discloses a two-step continuous process of producing low alkyl ester using alcohol and oil/fat in the presence of a liquid catalyst. According to this patent ('820), the pre-heated oil/fat reacts with alcohol in the presence of a catalyst solution in a plug flow reactor to produce glycerine and ester, and glycerine thus produced is separated from ester in the first step. In the second step, alcohol reacts again with the oil/fat in the catalyst solution in the plug flow reactor to produce ester and glycerine, and glycerine is separated from ester to produce low alkyl ester.
In this respect, because alcohol is immiscible with the oil/fat, it is required to maintain a Reynolds number to be 10,000 or higher so as to enable a mixture of alcohol and oil/fat to flow in a form of a turbulent flow instead of a laminar flow through the reactor to increase the reaction rate of alcohol with oil/fat. However, a length of the plug flow reactor must be sufficiently long, or a flow rate of the mixture of alcohol and oil/fat must be desirably high in order to enable the mixture to flow in the form of the turbulent flow through the plug flow reactor causing the enlargement in volume of the reactor and the increase in power costs, thereby undesirably increasing the production costs of low alkyl ester. In addition, the above patent is problematic in that a separation time of glycerine from alkyl ester is undesirably long because liquid drops of the product are finely broken during a settling process of separating glycerine from alkyl ester.
Furthermore, Canadian Pat. Laid-Open Publication No. 2,131,654 (Mar. 9,1996) recites a process of reacting fatty acid glyceride with alcohol in the presence of a catalyst using a subsidiary solvent to produce low alkyl fatty acid ester.
According to the patent ('654), fatty acid glyceride and alcohol are immiscible with each other if they are mixed with each other without sufficient agitation. Hence, in the case of the reaction of alcohol and fatty acid glyceride without sufficient agitation, the contact area between alcohol and fatty acid glyceride is relatively small, and the reaction rate is relatively slow because the reaction of alcohol and fatty acid glyceride is achieved at only an interface between them. In the above patent ('654), the subsidiary solvent, such as tetrahydrofuran (THF) and 1,4-dioxane, is added to reactants so as to increase the contact area between alcohol and fatty acid glyceride to increase the reaction rate, thereby leading to a homogeneous liquid mixture of alcohol and fatty acid glyceride to quickly convert the two phases into a single phase to sufficiently increase the reaction rate even though the reactants are in a laminar flow region with a low agitation speed. However, the patent ('654) has a disadvantage in that the subsidiary solvent must be separated from the product after the completion of the reaction of fatty acid glyceride with alcohol leading to the additional costs of installation and operation of a subsidiary solvent separating device, thereby ensuring low economic efficiency.
Meanwhile, Austrian Pat. Laid-Open Publication No. PCT/AT 98/00284 (International Publication No. WO 99/26913) discloses a method and a device for producing fatty acid methyl ester.
In this respect, an interface area between reactants is enlarged due to a high energy turbulent flow or a dynamic turbulent flow. However, the above Austrian patent is dis-advantageous in that the enlarged interface area is obtained by use of an additional agitator, which also has the same disadvantages as the patents described above.